In the manufacture of cigarettes, tobacco leaf is processed to separate the stems from the lamina. The lamina are shredded and formed into cigarettes or other smoking articles. The stems are not successfully utilizable as such in cigarette making because of their relatively large diameter, their hard nature and poor burning properties.
Nevertheless, tobacco stem material constitutes a substantial proportion of the leaf, usually about 20 to 25% of the weight thereof, and contains tar, nicotine and other materials common to the lamina. In the past, therefore, efforts have been made to process the stem material for use in smoking articles but such procedures have generally been unsatisfactory, unsuccessful or inefficient.
One prior art procedure which has been adopted, involves passing the stems, usually after moistening to about 30 t.% moisture, between rollers which act to crush the stems into sheet material, and cutting the sheet material into shreds for mixing with shredded lamina from which the smoking article is made. The product of this operation is commonly termed "cut rolled stem" (CRS). Cut rolled stem suffers from the drawbacks that it has only limited filling power, i.e., it has a limited ability to fill a cigarette tube, and hence more material is required to be present in the cigarette tube to achieve the same hardness of cigarette than for a higher filling power material.
A variation of this prior art procedure involves soaking and fast drying of the cut rolled stem, which produces a product commonly known as "enhanced cut rolled stem" which has an improved filling power as compared with the cut rolled stem.
Another prior art procedure is described in U.S. Pat. No. 4,094,323 assigned to American Brands Inc. wherein the stem material first is conditioned at a temperature of 115.degree. to 170.degree. C. under a superatmospheric pressure and thereafter is fiberized and expanded using a pressurized shredder comprising closely-spaced counter-rotating ribbed plates at an elevated temperature of about 115.degree. to 170.degree. C. This procedure produces a fiberized and expanded stem material having a substantially improved filling power when compared with cut rolled and enhanced cut rolled stem.
However, this latter prior art procedure also produces a considerable proportion of fine particulate material or "dust", as determined by passage through an 18-mesh sieve, usually about 30 wt.%, which is less suitable for use directly in cigarette making. The particulate material may be separated from the fiberized expanded stem prior to utilization of the latter and may be used to form reconstituted tobacco sheet, but such a procedure is not always available and also means additional processing. The properties ascribed to the fiberized stem material in U.S. Pat. No. 4,094,323 are determined after separation of the fine particulate material.
In addition, while the fiberized and expanded stem material have a substantially improved filling power, when compared with CRS or enhanced CRS, other properties of the product are less satisfactory from the point of view of utility. The burn rate of the material is substantially increased with respect to CRS and enhanced CRS, meaning that, while a lesser combined weight of shredded tobacco lamina and stem may be used in the cigarette for the same overall hardness as a blend of shredded tobacco lamina and CRS, nevertheless, the increase in burning rate of the cigarette which results from the lesser overall amount of tobacco and the increased burning rate of the fiberized and expanded stem material is detrimental.
Furthermore, the pressure drop which results along a cigarette made therefrom is substantially greater than for a cigarette made from enhanced CRS which in turn is greater than for a cigarette made from CRS, under the same cigarette conditions. The pressure drop along the cigarette relates to the ability of the smoker to draw smoke from the cigarette into his mouth, and lower values are generally considered more satisfactory than higher values.